Recently, various enterprises and universities are actively engaged in the development of electronic paper. The most promising application of electronic paper is electronic books, and other applications include the field of portable apparatus such as sub-displays of mobile terminal apparatus, and display sections of IC cards. One type of display devices used for electronic paper is liquid crystal displays utilizing a liquid crystal composition forming a cholesteric phase (such a composition is referred to as “cholesteric liquid crystal” or “chiral nematic liquid crystal”, and the term “cholesteric liquid crystal” will hereinafter be used). A cholesteric liquid crystal has excellent features such as semi-permanent display retention characteristics (capability of displaying an image when no electric power is supplied; memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
FIG. 20 schematically shows a sectional configuration of a liquid crystal display 51 capable of full-color display utilizing cholestric liquid crystals. The liquid crystal display 51 has a structure in which a liquid crystal display element 46b for blue (B), a liquid crystal display element 46g for green (G), and a liquid crystal display element 46r for red (R) are formed one over another in the order listed from the side of the display where a display surface is provided. In the illustration, the display surface is located on the side of a top substrate 47b, and external light (indicated by the arrow in a solid line) impinges on the display surface from above the substrate 47b. An eye of a viewer and the viewing direction of the viewer (indicated by the arrow in a broken line) are schematically shown above the substrate 47b. 
The B liquid crystal display element 46b includes a blue (B) liquid crystal 43b enclosed between a pair of substrates, i.e., a top substrate 47b and a bottom substrate 49b and a pulse voltage source 41b for applying a predetermined pulse voltage to the B liquid crystal 43b. The G liquid crystal display element 46g includes a green (G) liquid crystal 43g enclosed between a pair of substrates, i.e., a top substrate 47g and a bottom substrate 49g and a pulse voltage source 41g for applying a predetermined pulse voltage to the G liquid crystal 43g. The R liquid crystal display element 46r includes a red (R) liquid crystal 43r enclosed between a pair of substrates, i.e., a top substrate 47r and a bottom substrate 49r and a pulse voltage source 41r for applying a predetermined pulse voltage to the R liquid crystal 43r. Although not shown, a plurality of electrodes are formed on an interface of the each of the top substrates 47 and bottom substrates 49 in contact with the liquid crystal 43 to apply the pulse voltage to the liquid crystal 43 from the respective pulse voltage source 41. A light absorbing layer 45 is provided on a bottom surface of the bottom substrate 49r of the R liquid crystal display element 46r. 
The cholesteric liquid crystal used as each of the B, G, and R liquid crystals 43b, 43g, and 43r is a liquid crystal mixture obtaining by adding a relatively great amount of chiral additive (also referred to as “chiral material”) to a nematic liquid crystal to a content of several tens percent by weight. When a nematic liquid crystal includes a relatively great amount of chiral material, a cholesteric phase, which is a great helical twist of nematic liquid crystal molecules, can be formed in the liquid crystal. For this reason, a cholesteric liquid crystal is also referred to as “chiral nematic liquid crystal”.
A cholesteric liquid crystal has bi-stability (memory characteristics), and the liquid crystal can be put in any of a planar state, a focal conic state, or an intermediate state which is a mixture of the planar state and the focal conic state by adjusting the intensity of an electric field applied to the same. Once the liquid crystal enters the planar state, the focal conic state, or the mixed or intermediate state, the state is thereafter kept with stability even after the electric field is removed.
The planar state can be obtained by applying a predetermined high voltage between a top substrate 47 and a bottom substrate 49 to apply a strong electric field to the liquid crystal 43 and to thereby reset the liquid crystal 43 to the homeotropic state and thereafter nullifying the electric field abruptly. For example, the focal conic state can be obtained by applying a predetermined voltage lower than the above-described high voltage between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal 43 and thereafter nullifying the electric field abruptly.
For example, the intermediate state which is a mixture of the planar state and the focal conic state can be obtained by applying a voltage lower than the voltage to obtain the focal conic state between the top substrate 47 and the bottom substrate 49 to apply an electric field to the liquid crystal 43 and thereafter nullifying the electric field abruptly.
A display principle of the liquid crystal display 51 utilizing cholesteric liquid crystals will now be described by referring to the B liquid crystal display element 46b as an example. FIG. 21A shows alignment of liquid crystal molecules 33 of the B liquid crystal 43b of the B liquid crystal display element 46b observed when the liquid crystal is in the planar state. As shown in FIG. 21A, in the planar state, the liquid crystal molecules 33 are sequentially rotated in the thickness direction of the substrates to form helical structures, and helical axes of the helical structures are substantially perpendicular to substrate surfaces.
In the planar state, light in a predetermined wave band in accordance with the helical pitch of the liquid crystal molecules 33 is selectively reflected by the liquid crystal layer. The reflected light is circularly polarized light which is either left- or right-handed depending on the chirality of the helical pitches, and other types of light are transmitted by the liquid crystal layer. Natural light is a mixture of left- and right-handed circularly polarized light. Therefore, when natural light in the predetermined wave band impinges on the liquid crystal in the planar state, it may be assumed that 50% of the incident light is reflected with the other 50% transmitted.
A wavelength λ at which maximum reflection takes place is given by λ=n·p where n represents the average refractive index of the liquid crystal and p represents the helical pitch.
Therefore, in order to allow blue light to be selectively reflected by the B liquid crystal 43b of the B liquid crystal display element 46b in the planar state, the average refractive index n and the helical pitch p are determined, for example, such that an equation “λ=480 nm” holds true. The average refractive index n can be adjusted by selecting the liquid crystal material and the chiral material appropriately, and the helical pitch p can be adjusted by adjusting the chiral material content.
FIG. 21B shows alignment of the liquid crystal molecules 33 observed when the B liquid crystal 43b of the B liquid crystal display element 46b is in the focal conic state. As shown in FIG. 21B, in the focal conic state, the liquid crystal molecules 33 are sequentially rotated in an in-plane direction of the substrates to form helical structures, and helical axes of the helical structures are substantially parallel to the substrate surfaces. In the focal conic state, the B liquid crystal 43b loses the selectivity of wavelengths to be reflected, and most of incident light is transmitted by the layer. Since the transmitted light is absorbed by the light absorbing layer 45 disposed on the bottom surface of the bottom substrate 49r of the R liquid crystal display element 46r, a dark state (black) can be displayed.
In the intermediate state that is a mixture of the planar state and the focal conic state, the ratio between reflected light and transmitted light is adjusted according to the ratio of presence between the planar and focal conic states, and the intensity of reflected light varies accordingly. Therefore, multi-gray-level display can be performed according to intensities of reflected light.
As described above, the quantity of light reflected by the cholesteric light crystal can be controlled by a helically twisted state of alignment of liquid crystal molecules 33. Cholesteric liquid crystals selectively reflecting green and red light rays in the planar state are used as the G liquid crystal 43g and the R liquid crystal 43r, respectively just as done for the B liquid crystal 43b to fabricate the liquid crystal display 51 capable of full-color display. The liquid crystal display 51 has memory characteristics, and it is capable of performing full-color display without consuming electric power except during a screen rewrite.
Although a reflectance spectrum of a cholesteric liquid crystal has a distribution having a well-regulated shape like a normal distribution, the peak of the reflectance of the liquid crystal has an upper limit of 50% as described above. The image displaying quality of the liquid crystal display 51 is therefore still lower than that of printed matters such as paper. It is considered that display quality higher than that of printed matters can be achieved by raising the peak of reflectance into the range between 70 to 80% while maintaining such a well-regulated distribution of a reflectance spectrum. Methods proposed as approaches toward improved reflectance include the use of a six-layer structure in which an R-enantiomer and an L-enantiomer (S-enantiomer) are used in each of R, G, and B liquid crystals. However, the method is impractical because the use of a simple six-layer structure may result in an increase in manufacturing cost which overwhelms the advantage of improved image quality.
Common display elements display an image on one surface thereof with a light absorbing layer (black layer) disposed on a rearmost surface thereof, and electronic books and guide plates presently available for practical use also display an image on one surface thereof. However, when a display element is used as a substitute for a book such as an electronic book, inconvenience is encountered, for example, in that a user cannot return to the previous page quickly to reread the same. In consideration to such a situation, an electronic book allowing page-flipping has been proposed in JP-A-2000-292777, the electronic book having pages on both sides thereof constituted by display layers employing cholesteric liquid crystals. In this case, however, since images are displayed on both sides using a structure having three layers, i.e., R, G, and B layers, the electronic book encounters a limit in image quality when displaying contents which must have high image quality such as photographs.